A blood pump 2 comprising: a cartridge 4 having a concave recess 8 therein having a surface, and a flexible membrane 0 covering said recess, the concave recess and flexible membrane forming a pump chamber 12 having an inlet 14 and an outlet 16 wherein: said flexible membrane is movable between a first position, separated from said surface wherein in such position the pump chamber has a maximum volume, and a second position, substantially adjacent said surface such that in said second position the pump chamber has a minimum volume, a pump driver mechanism 18 arranged to interface with the cartridge, said driver operable to move the flexible membrane in a first direction into the recess to, in use, pump blood from the chamber and operable to move the flexible membrane in a second direction away from the recess to, in use, draw blood into said chamber and wherein the pump driver controls the movement of the flexible membrane in the first direction such that the speed at which it is moving reduces as it approaches the surface of the concave recess.
